Underground drilling, such as gas, oil, or geothermal drilling, generally involves drilling a bore through a formation deep in the earth. Such bores are formed by connecting a drill bit to long sections of pipe, referred to as a “drill pipe,” so as to form an assembly commonly referred to as a “drill string.” The drill string extends from the surface to the bottom of the bore.
The drill bit is rotated so that the drill bit advances into the earth, thereby forming the bore. In rotary drilling, the drill bit is rotated by rotating the drill string at the surface. Piston-operated pumps on the surface pump high-pressure fluid, referred to as “drilling mud,” through an internal passage in the drill string and out through the drill bit. The drilling mud lubricates the drill bit, and flushes cuttings from the path of the drill bit. In the case of motor drilling, the flowing mud also powers a drilling motor which turns the bit, whether or not the drill string is rotating. The drilling mud then flows to the surface through an annular passage formed between the drill string and the surface of the bore.
The drilling environment, and especially hard rock drilling, can induce substantial vibration and shock into the drill string. Vibration also can be introduced by factors such as rotation of the drill bit, the motors used to rotate the drill string, pumping drilling mud, imbalance in the drill string, etc. Such vibration can result in premature failure of the various components of the drill string. Substantial vibration also can reduce the rate of penetration of the drill bit into the drilling surface, and in extreme cases can cause a loss of contact between the drill bit and the drilling surface.
Operators usually attempt to control drill string vibration by varying one or both of the following: the rotational speed of the drill bit, and the down-hole force applied to the drill bit (commonly referred to as “weight-on-bit”). These actions are frequently in reducing the vibrations. Reducing the weight-on-bit or the rotary speed of the drill bit also usually reduces drilling efficiency. In particular, drill bits typically are designed for a predetermined range of rotary speed and weight-on-bit. Operating the drill bit away from its design point can reduce the performance and the service life of the drill bit.
So-called “shock subs” are sometimes used to dampen drill string vibrations. Shock subs, however, typically are optimized for one particular set of drilling conditions. Operating the shock sub outside of these conditions can render the shock sub ineffective, and in some cases can actually increase drill string vibrations. Moreover, shock subs and isolators usually isolate the portions of the drill string up-hole of the shock sub or isolator from vibration, but can increase vibration in the down-hole portion of the drill string, including the drill bit.
One approach that has been proposed is the use of a damper containing a magnetorheological (hereinafter “MR”) fluid valve. The viscosity of MR fluid can be varied in a down-hole environment by energizing coils in the valve that create a magnetic field to which the MR fluid is subjected. Varying the viscosity of the MR fluid allows the damping characteristics to be optimized for the conditions encountered by the drill bit. Such an approach is disclosed in U.S. Pat. No. 7,219,752, entitled System And Method For Damping Vibration In A Drill String, issued May 22, 2007, hereby incorporated by reference in its entirety.
The aforementioned U.S. Pat. No. 7,219,752 discloses an MR valve using a mandrel to hold the coils that is made of 410 martensitic stainless steel. Prior art embodiments of similar MR valves have used coil holders made of 12L14 low carbon steel (which has a saturation magnetization of about 14,000 Gauss, a remnant magnetization of 9,000 to 10,000 Gauss, and a coercivity of about 2 to 8 Oersteds) and 410/420 martensitic stainless steel. The shafts in such embodiments have been made of 410 stainless steel, which can have a relative magnetic permeability of 750 Gauss and a coercivity of 6 to 36 Oe. Unfortunately, the inventors have found that the minimum level of damping achievable using such MR valves is compromised by the fact that energizing the coil can result in a low level of permanent magnetization of the valve components. Although this residual, or remnant, magnetization is considerably below that normally used to provide effective damping, it reduces the range of the MR fluid viscosity at the lower end and, therefore, the minimum damping that can be obtained. In prior art MR valves, the problem of remnant magnetization has been addressed by demagnetizing components of the valve that had become permanently magnetized by supplying to the coils current of alternating polarity and decreasing amplitude in a stepwise fashion.
A problem experienced by prior art MR valves is that using a coil to maintain the magnetic field requires a considerable amount of electrical energy. Consequently, turbine alternators, which are expensive and costly to maintain, are typically required to power the coils. An ongoing need, therefore, exists for a MR fluid damping system that can dampen drill-string vibrations, and particularly vibration of the drill bit, throughout a range of operating conditions, including high and low levels of damping, that does not require large amounts of electrical energy.